That's €60 billion higher than at the start of the crisis in 2008, and means the submerged economy was worth a whopping 24.6 percent of GDP in 2012.

In 2008, by contrast, this underground activity was valued at a still-high 17.8 percent of GDP.

The Gestha report ties the massive growth in Spain's submerged economy to high levels of corruption, the lingering effects of the country's building boom — which created a huge pool of black money — and massive unemployment.

The union also pointed out that tax increases brought in since the start of the crisis had not been adequately policed by authorities.

Another key factor was the massive circulation of €500 notes in the Spanish economy. This amounts to some 73.7 percent of all currency moving around the country, or 14 percent of all such notes in use in Europe, according to Gestha.

These €500 notes are the preferred bills for underground activity said the union.

On a region-by-region breakdown, the areas hardest hit by the collapse of the building boom and unemployment including Andalusia, the Canary Islands, Extremadura, and Castile–La Mancha had the highest levels of black activity.

The lowest levels were seen in the provinces of Madrid, Tarragona, Lleida, Barcelona, Zaragoza and La Rioja.